The relationship between the structure and the function of sterols will be examined using the following biological systems: (a) those in which sterols are not biosynthesized (insects, protozoa, and anaerobically grown yeast) and (b) those in which sterols are normally formed, e.g., mammals. The first group permits an opportunity to examine membranous function and immediate metabolism, while the second group allows such things as the full biosynthetic process and feedback regulation to be examined. The principal methodology will be to synthesize particular sterols, incubate them, and determine the result, such as extent of growth support or metabolism, on a quantitative basis with respect to the sterol used as substrate. The natural occurrence of various sterols will also be studied as a function of taxon.